Safety and Efficacy Of Long-Term Open-Label Romiplostim Dosing In Thrombocytopenic Pediatric Patients With Immune Thrombocytopenia (ITP)

Background Chronic pediatric ITP is an autoimmune disorder characterized by increased platelet destruction and suboptimal platelet production, resulting in low platelet counts. Romiplostim is a peptibody that stimulates platelet production via activation of the thrombopoietin (TPO) receptor. Romiplostim increased and maintained platelet counts in thrombocytopenic children with ITP in a phase 1/2 trial. Patients who completed this study or an ongoing phase 3 trial were given the option of rolling over into an open-label long-term extension study. Objectives To evaluate the safety and efficacy of long-term use of romiplostim in pediatric ITP. Methods Patients received weekly subcutaneous injections of romiplostim; the starting dose was the last dose in the prior study. Patients who had received placebo started at 1 μg/kg. Dose adjustments targeted platelet counts in the range of 50–200 x 109/L. The maximum allowed romiplostim dose was 10 µg/kg. Assessments of adverse events, concomitant medications, and local platelet counts were performed weekly. The primary endpoint was incidence of adverse events. The protocol did not require bone marrow biopsies to be performed, but when performed, specimens were submitted to a central lab for analysis. Patients who were on a stable dose had the option to receive romiplostim at home; patients and their caregivers then recorded dosing date, time, volume administered, and any dosing errors. Patients who turned 18 years of age during this study were permitted to remain on study. Results Twenty-two patients (N = 12 from the phase 1/2 study and N = 10 from the phase 3 study) were treated with romiplostim for up to 172 weeks (3.3 years). Baseline demographics included a median age of 12.0 years (range 3–16), 50% male, and 18.2% with prior splenectomy. Median romiplostim treatment duration was 89.0 weeks (range 3–172); median total number of doses was 64 (range 3–171); median average weekly romiplostim dose was 4.0 µg/kg (range 1–10), including ramp up to stable dose; and median maximum dose was 8.0 µg/kg (range 1–10). Of the 4 patients who discontinued the study, 3 withdrew consent and 1 was noncompliant; no patients withdrew due to safety issues and 18 continued on study. After the first week of this extension study, which for some patients was the first week of romiplostim, median platelet counts remained above 50 x 109/L throughout the study, and were in the target range of 50–200 x 109/L for all visits but weeks 76 and 156 (Figure). The median romiplostim dose (Q1, Q3) was 6.0 (2.0, 8.0) µg/kg at week 1 and 3.5 (0.0, 7.0) µg/kg at week 168 (Figure). Four patients discontinued romiplostim. Eight patients received rescue medications (defined as medications used for platelet counts < 10 x 109/L, bleeding/wet purpura, or investigator decision) which included immunoglobulins (3 patients), tranexamic acid (3 patients), platelet transfusion (1 patient), aminocaproic acid (1 patient), and prednisone (1 patient). Four patients had serious adverse events (asthma, hemangioma, hypotension, infection, thrombocytopenia, and transfusion reaction) and 1 had life-threatening adverse events (infection and thrombocytopenia). None of the serious adverse events were deemed treatment-related by the investigators. There were no fatal adverse events. Twelve patients had bleeding adverse events; 2 of which were deemed treatment-related (gingival bleeding and petechiae). Bleeding adverse events included epistaxis (4 patients); petechiae (3 patients); gingival bleeding (2 patients); hemorrhage (2 patients); and bleeding from the anus, injection site, lip, and mouth (1 patient each). No bone marrow biopsies were performed as part of this study. Conclusion In this open-label extension study, long term treatment with romiplostim maintained platelet counts in pediatric patients with chronic ITP without significant toxicity. Future results from this ongoing study will provide additional safety and efficacy data regarding long-term use of romiplostim in children with ITP. Disclosures: Tarantino: Pfizer: Membership on an entity’s Board of Directors or advisory committees; Octapharma: Membership on an entity’s Board of Directors or advisory committees; Novo Nordisk: Membership on an entity’s Board of Directors or advisory committees, Research Funding; Grifols: Membership on an entity’s Board of Directors or advisory committees, Research Funding, Speakers Bureau; BPL: Membership on an entity’s Board of Directors or advisory committees; Baxter: Membership on an entity’s Board of Directors or advisory committees; Amgen: Membership on an entity’s Board of Directors or advisory committees. Off Label Use: Romiplostim is approved for the treatment of adults with chronic ITP. Romiplostim is not approved for the treatment of pediatric patients with chronic ITP. Bussel:Amgen: Equity Ownership, Membership on an entity’s Board of Directors or advisory committees, Research Funding; GlaxoSmithKline: Equity Ownership, Membership on an entity’s Board of Directors or advisory committees, Research Funding; Cangene: Research Funding; Genzyme: Research Funding; IgG of America: Research Funding; Immunomedics: Research Funding; Ligand: Membership on an entity’s Board of Directors or advisory committees, Research Funding; Eisai: Membership on an entity’s Board of Directors or advisory committees, Research Funding; Shionogi: Membership on an entity’s Board of Directors or advisory committees, Research Funding; Sysmex: Research Funding; Symphogen: Membership on an entity’s Board of Directors or advisory committees. Nie:Amgen: Employment, Equity Ownership. Eisen:Amgen: Employment, Equity Ownership.